Virus vector has widely been studied for efficiency of vaccination and for use as a gene introduction system into the living body. Particularly in the poultry industry, a study for application of poxviruses, especially fowlpox virus, has made a rapid progress. Under the circumstances, in order to develop a more efficient vector, the present inventors have investigated to make a vector from Marek's disease virus type 1 (hereinafter also referred to as "MDV1"), a kind of avian herpes viruses, and have reported many results thereof. For example, the present inventors have investigated more than 20 sites on the MDV1 genome, and as a result, have identified US10 gene as an insertion site for an exogenous gene which can stably retain an exogenous gene but does not impair the vaccine effects against Marek's disease [Japanese patent application No. 4-205933 (Japanese patent publication No. 6-22757); 4th International Symposium on Marek's Disease (1992), Amsterdam; Vaccine 1994 Vol. 12, 953-957]. A recombinant virus which incorporates F protein gene of Newcastle disease virus into the US10 gene exhibited sufficient effects as a vaccine in SPF chickens, said effects being persistent over at least 24 weeks after inoculation (4th International Symposium on Marek's Disease (1992), Amsterdam). This recombinant virus has been proved to show protective effects against both Marek's disease and Newcastle disease even in chickens having maternal antibodies but the protective effects were somewhat lowered than in SPF chickens. That is, this recombinant virus showed efficiently 100% protective effects against Newcastle disease in SPF chickens whereas it showed somewhat lowered effects, i.e. 70 to 90% level, in field chickens having maternal antibodies (Current Developments in the Molecular Biology of Marek's Disease Virus Workshop, 1995, Florida). Such a decrease in the effects was supposedly due to suppression of in vivo growth of the recombinant virus by maternal antibodies against Newcastle disease and Marek's disease when field chickens are immunized with the recombinant virus.